Im assuming its some kind of general knowledge test thing, like 7 DIAW = 7 "days in a week"?

If the letters are sequential on purpose then they must represent either a list of items that are normally ranked (say, worlds tallest peaks or planets of the solar system listed in order of distance from the sun) or they are initials of words that are sequential in time or usage, say Kings of England (time) or Richard Of York Gave Battle In Vain (usage).

It wont be a "group" of words with no natural order. (unless you have mistakenly said "sequence" instead of "which one is missing")

But ATM I cant think of the answer, so Im no sodding use at all, sorry

Given that it's from a school and therefore shouldn't be too difficult, yet it's legged up the Internet Authority On Everything forum, I'd hazard that either it's referring back to some lesson your kid's had that she's not told you about or it's specific to the school / area somehow.